1. Field of the Invention
The present invention relates to a device for and a method of detecting interference waves that can interfere with the reception of radio signals.
2. Description of the Prior Art
Referring next to FIG. 11, there is illustrated a block diagram showing the structure of a prior art interference wave detecting device disposed at a base station, as disclosed in for example Japanese Patent Application Laying Open (KOKAI) No. 62-166627. In the figure, reference numeral 11 denotes a local oscillator for generating a transmission local oscillating signal, 2 denotes a transmitter for modulating data to be transmitted by the base station and for mixing the modulated signal and the transmission local oscillating signal so as to generate a radio signal of a certain transmission frequency, and 3 denotes a transmission/reception branching circuit for furnishing the radio signal from the transmitter 2 to a transmitting/receiving antenna 4 and for furnishing a radio signal applied thereto, which has been transmitted by a mobile station, by way of the transmitting/receiving antenna 4 to a receiver 5. The receiver 5 can mix the radio signal from the transmission/reception branching circuit 3 and a reception local oscillating signal so as to convert the frequency of the radio signal into a certain frequency and demodulate the radio signal to extract received data. A detector 5 can detect the level of the radio signal received by the receiver 5.
In addition, reference numeral 7 denotes a second local oscillator for generating a second reception local oscillating signal, 8 denotes a receiving antenna for receiving a radio signal having a certain frequency different from that of the radio signal from the mobile station received by the receiver 5, 9 denotes a second receiver for mixing the radio signal received by the receive antenna 8 and the second reception local oscillating signal so as to convert the frequency of the radio signal into a certain frequency and demodulate the radio signal to extract received data, 10 denotes a second detector for detecting the level of the radio signal received by the second receiver 9, 11 denotes a switching coupler for selecting either the first received data from the first receiver 5 or the second received data from the second receiver 9 and for adopting the selected data as data received by the base station, and 12 denotes a controller for controlling the switching coupler 11 and so on.
When the base station transmits or sends data to a mobile station, the transmitter 2 modulates a carrier wave with the data to be transmitted and then mixes the modulated signal and the first local oscillating signal from the first local oscillator 1 so as to convert the modulated data to be transmitted into a radio signal of a certain transmission frequency. The transmitter 2 then transmits the radio signal to the mobile station by way of the transmission/reception branching circuit 3 and the transmitting/receiving antenna 4.
On the other hand, when the base station receives a radio signal transmitted by a mobile station, the first receiver 5 mixes the radio signal applied thereto by way of the transmitting/receiving antenna 4 and the first reception local oscillating signal so as to convert the frequency of the received radio signal into a certain frequency and demodulate the radio signal to extract received data. The switching coupler 11 then adopts the extracted data as data received by the base station.
The level of the radio signal received by the first receiver 5 is not always constant. Interference waves having the same frequency as the received radio signal can reduce line quality. It is therefore necessary to change the predetermined reception frequency that the received radio signal has to have when the level of the received radio signal is reduced.
To this end, conventionally, the first detector 6 detects the level of the radio signal received by the first receiver 5, and the controller 12 then monitors the detected level and changes the predetermined reception frequency that the received radio signal has to have if the detected level is lower than a predetermined level, as will be described below. In order to tune from the current reception frequency, i.e. the current upward channel, to another reception frequency, i.e. another channel, at which the level of interference waves can be minimized, the controller 12 measures the level of interference waves having the other reception frequency in advance of changing the current reception frequency.
As previously mentioned, while the first receiver 5 receives a radio signal transmitted by a mobile station, the second receiver 9 receives and converts another radio signal lying having a predetermined frequency which is different from that of the radio signal being received by the first receiver 5 into a signal having another certain frequency. The second detector 10 detects the level of the other radio signal received by the second receiver 9.
The controller 12 then compares the detected level of the other radio signal received by the second receiver 9 with an acceptable level of interference waves. If the detected level of the other radio signal received by the second receiver 9 is lower than accepted, it is assumed that the level of interference waves having the predetermined frequency is low. Thus the controller 12 realizes that the predetermined frequency is an acceptable reception frequency to which the current reception frequency can be changed.
If the detected level of the radio signal received by the first receiver 5 is lower than a predetermined level, the controller 12 changes the current reception frequency that the received radio signal has to the above-mentioned acceptable reception frequency. After that, the controller 12 instructs the switching coupler 11 to select or adopt the second received data from the second receiver 9 as data received by the base station.
On the other hand, if the detected level of the other radio signal received by the second receiver 9 is higher than accepted, it is assumed that the level of interference waves having the predetermined frequency is high. In this case, the controller 12 instructs the second local oscillator 7 to change the frequency of the second reception local oscillating signal so as to change the predetermined reception frequency, i.e. the current reception frequency adopted by the second receiver 9.
After that, the controller 12 repeats the above mentioned processes so as to search for a desired reception frequency at which the level of interference waves is lower than accepted.
While the prior art interference wave detecting device so constructed as mentioned above can tune from the current reception frequency adopted by either the first receiver 5 or the second receiver 9 to another reception frequency at which the level of interference waves is low, it suffers from a disadvantage that it can improve the line quality of the upward line from the mobile station to the base station, but cannot improve the line quality of the downward line from the base station to the mobile station, because it does not have a function of detecting the level of interference waves having the same frequency as the radio signal being transmitted by the transmitter 2, that is, because it cannot independently detect the level of interference waves having the same frequency as the radio signal being transmitted by the transmitter 2 by simply monitoring the level of the radio signal from the transmitter 2.
Although the provision of another receiver intended for receiving the radio signal being transmitted by the transmitter 2 makes it possible to monitor the level of the radio signal received, it does not make it possible to independently detect the level of interference waves having the same frequency as the radio signal being transmitted by the transmitter.